1. Field of the Invention
The present invention relates to methods for making highly reflective metal and in particular to a method of making reflective aluminum sheet and to making brightened aluminum trim for use in automobiles, trucks, boats and a variety of household and industrial appliances.
2. Description of the Related Art
Steel sheet with a silvered polymer film laminated to it, and formed to a desirable shape, has gained wide market acceptance for use in lighting fixtures where high reflectance is critical and cost is a secondary consideration, as for example, for light in hospital operating rooms. Relatively less expensive lighting fixtures are made from mild steel painted with a paint containing a white opaque powder having high total reflectance but low distinctness of (reflected) image ("D/I" for brevity). Narrow polished, bright sheets (referred to as "strips") of stainless steel and/or stainless steel clad aluminum (referred to as "bi-metal"), appropriately shaped, are also widely used for decorative trim in automobiles, trucks, boats and a variety of both household and industrial appliances because such decorative trim is eminently durable under aggressive conditions of use. The cost of stainless steel sheet has provided the impetus to replace decorative stainless steel trim with brightened aluminum trim.
The problem is that a brightened, coated and shaped reflective aluminum strip, provided with the protection afforded by any one or more of known coatings, whether inorganic or organic, or both, fails to meet numerous tests which are deemed essential if aluminum trim is to be substituted for the polished stainless steel trim.
This invention relates generally to a shaped, aluminum article having substantially minor-like characteristics, formed by continuously shaping a "strip" of fluoropolymer-coated aluminum alloy, for example, in a roll-forming die, which provides the strip with at least one "tight" radius which is less than 10 mm (0.375 inch). By "substantially mirror-like characteristics" is meant that the surface is characterized by having at least 75% and preferably at least 80% D/I. D/I is the sharpness of the reflected image as measured by the ratio of the reflectance at 0.3.degree. from specular to the reflectance at the specular angle, that is, EQU D/I=[(R.sub.S -R.sub.0.3)/R.sub.S ].times.100%
R.sub.S =specular reflectance; D/I=0 for a perfect diffuser; D/I=100 for a perfect mirror. Total reflectance of a surface is irrelevant in a consideration of its D/I.
It is well known that chemical treatments are used to remove soiled and oxidized aluminum surfaces, to brighten them to a specular luster, and to develop various types of protective or decorative coatings. The greatest value of a chemical treatment is as a pretreatment for applying finishes, including organic coatings and laminates, anodizing, electroplating, etc. The adhesion of these finishes, and others, depends in great measure on the type and quality of the chemical pretreatment. A chemical pretreatment may be outstanding as a preparation for paint, but inadequate as a pretreatment for another finish. The result is that, over the years, hundreds of chemical treatments and finishes have been developed to meet diverse needs. (See Aluminum Vol. III. Fabricating and Finishing, edited by Kent R. Van Horn, Chapter titled "Chemical Pretreating and Finishing" by George, D. J. et al. page 587 American Society for Metals, Metals Park, Ohio).
Faced with the problem of making a highly reflective aluminum surface, one skilled in the art typically chooses an aluminum alloy with a known propensity to acquire and retain a high specular luster after being mechanically bright-rolled in coil form. If one starts with such an alloy, it is mechanically bright-rolled to a high luster and cleaned.
Among numerous choices of highly reflective aluminum alloys is the use of one containing from 0.5-3% magnesium, from 0.2-0.5% silver, from 0.001-0.2% iron and from 0.01-0.15% silicon (see U.S. Pat. No. 3,720,508 to Brock et al, class 75/147): and an alloy consisting essentially of 0.25-1.5% Mg (see U.S. Pat. No. 4,601,796 to Powers et al, class 204/33), the balance in each case being aluminum. Because essentially pure aluminum has excellent reflectance, by far the most popular choices for aluminum alloys are those with a low content of alloying elements. Such alloys have inadequate strength for numerous applications which also require a specular reflectance greater than 45%, often greater than 60%. As might be expected, high strength aluminum alloys are not typically chosen for use in high reflectance applications. Yet these alloys of the AA 5XXX and AA 6XXX series, particularly 5657, 5252, 5182 and 6306, are the alloys of special interest for use in this invention.
It is known that the surface of such alloys may be protected by various treatments including anodic oxidation, hydrothermal treatment or conversion coatings employing solutions which may contain chromic acid, chromates, phosphoric acid, phosphates and fluorides. Anodic oxidation, for example, in a sulfuric acid bath, has been the bath of choice since more than a score of years ago (when it was disclosed in U.S. Pat. No. 3,530,048 to Darrow class 204/58). A thinner and more compact coating was provided by the addition of a hydrophilic colloid to the surface during the anodizing step (see U.S. Pat. No. 3,671,333 to Mosier class 204/58). A sulfuric acid anodized coating was favored for a highly reflective coating as recently as five years ago (U.S. Pat. No. 4,601,796 to Powers et al class 204/33).
It is also known to provide as thin a coating as would provide protection without vitiating the specularity of the surface. However, thin oxide coatings of the prior art, no matter how produced on a highly reflective aluminum surface, are far too thick to withstand being sharply bent without "crazing", may provide adequate protection for a short time, but may not provide enough "texture" (familiarly referred to as "grab") to anchor a protective organic coating having excellent durability and optical properties. Further, a thin coating may craze when the strip of aluminum is bent over a 2.5 cm radius mandrel; an anodized coating not quite thin enough will also craze when bent to simulate a forming operation.
In the past, an electrolytic processing step in a phosphoric acid bath, after anodizing in a sulfuric acid bath, was used to provide a surface which was then electrocolored (see U.S. Pat. No. 4,022,671 to Asada class 204/42). But conversion coatings generally have a relatively low D/I because they tend to color the surface. Further, conversion coatings have typically provided less than satisfactory bond, for our purpose, with even the most preferred matrix fluoropolymer.
Another coating on aluminum which was produced with phosphoric acid anodizing followed by AC electrocoloring resulted in a surface with excellent optical properties, as disclosed in French Demande No. 2,360,051 to Showa Aluminum K.K. The process is carried out under constant current conditions of 1 to 1.5 amps/square decimeter. There is no indication as to how bright the sheet is after it is chemically cleaned, nor what the effects of the anodizing and coloring were. There is no indication whether any organic coating would adhere satisfactorily to the surface, least of all a fluoropolymer containing at least 40 mol % of fluoroolefin units, known to produce a cured film of matrix fluoropolymer most difficult to adhere to a smooth metal surface (see U.S. Pat. No. 4,070,525).
U.S. patent application Ser. No. 07/830,021, filed Jan. 31, 1992, discloses and claims a method for forming a reflective strip of aluminum by cleaning the surface to remove superficial contaminants, chemically or electrochemically brightening the cleaned surface, and anodizing the brightened surface. That application further discloses coating the anodized surface with a fluoropolymer to interstitially mechanically bond the fluoropolymer to the anodized surface. The fluoropolymer surface may be treated with corona discharge and a strip of thermoplastic polymer adhesively bonded to the treated surface.
A method is desired for producing reflective strip of aluminum having at least 80% D/I and which can be shaped into a profile having at least one small radius but which is less expensive to produce than is the strip of U.S. patent application Ser. No. 07/830,021.